When producing cellulose-based products, a roll press is frequently used for washing and dewatering the cellulose-based pulp. The pulp is passed between two cooperating press rolls installed in the roll press, the press rolls having a perforated outer surface, a so called mantle surface, whereby the outer surface is permeable to liquid pressed out of the pulp, and the pulp is pressed in the roll nip, or the press nip, between the press rolls, whereby liquid is pressed out of the pulp. The roll press also includes one or more washing zones upstream of the press nip. One example of such a roll press is disclosed in U.S. Pat. No. 3,980,518, in which the central axes of the press rolls lie in substantially the same horizontal plane, and the pulp passes through the press nip between the press rolls in a vertical direction, from below upwards.
The above-mentioned roll presses comprise a vat in which the two press rolls are installed and partially enclosed. The vat limits accessibility to the press roll, especially if the vat encloses a large portion of the circumference of the perforated outer surface of the press roll. Accessibility to the press roll, however, is necessary for performing maintenance work, cleaning the press roll and the press nip, e.g. clearing plugs and removing pulp jammed on the perforated outer surface or in proximity to the press roll, or for changing press rolls. U.S. Pat. No. 5,063,840 discloses several previously known solutions for providing accessibility to the press rolls and the press nip. In one known solution, the vat is provided with a number of small-sized vat access doors. In another known solution, a bottom door is provided, and the nip wedge, which converges towards the press nip, is an integral part of the bottom door. The bottom door is supported by a hoist cylinder and is movable in a vertical direction. However, the access provided by these previously known solutions is limited, and these doors do not provide sufficient working room for clearing plugs. To overcome these drawbacks, U.S. Pat. No. 5,063,840 suggests a solution where the vat is equipped with a bottom door having a nip wedge as an integral part, which bottom door is pivotally mounted by means of a hinge, whereby accessibility is increased.
The use of a vat having pivotable vat segments is also disclosed in U.S. Pat. No. 6,311,849 which describes an apparatus for dewatering and washing pulp, comprising two press rolls, between which the pulp is passed, the axes of rotation of the press rolls lying in a horizontal plane. The vat of this apparatus comprises two upper pivotable vat segments which can be folded upwards for providing better accessibility when performing maintenance work, especially when changing rolls. In this apparatus, the pulp is distributed on the press roll at the lowest position of the circumference of the outer surface.
Further, European Patent No. 1,035,250 describes apparatus for washing and dewatering pulp comprising two filter drums arranged to rotate in opposite directions to create a press nip. The filter drums are installed in a vat which comprises two pivotable upper vat segments for providing access to the filter drums. A gap is formed between the vat and the outer surface of the filter drums, in which gap the pulp is fed while being pressed and washed.
However, accessibility is still limited in these prior art solutions, especially when making inspections, changing press rolls, removing plugs, and cleaning the perforated outer surface or the region in proximity to the outer surface of the press roll, or the press nip.
One object of the present invention is thus to provide apparatus for washing and dewatering pulp which provides improved accessibility when performing maintenance work.